Most of the tritium on earth is present as an oxide like tritiated water, i.e., tritium water. Concentration of tritium water which circulates in air is believed to have almost a constant value in animals and plants of all ages in any area. From the reduced amount of concentration in water, a period separated from the air circulation can be determined, and dating of underground water can be also made. Concentration of tritium water is also used for actual investigation of a flow of underground water in the field of civil engineering and agriculture. Tritium is mixedly present in water as tritium water including oxygen, and it is widely present in water resources including water vapor, rainfall, underground water, stream water, lake water, sea water, drinking water, and a living organism as a gas phase, a liquid phase, or a solid phase.
Natural tritium is produced by a reaction between cosmic ray and air. However, due to low production probability, its amount is extremely small. Meanwhile, the tritium produced by the nuclear test in 1950s, a nuclear reactor and the reprocessing of nuclear fuel has been discharged and present in large amount in an environment (fallout tritium). Furthermore, compared to an external system, tritium produced during operation or maintenance of the reactor or reprocessing of nuclear fuel is accumulated and localized at higher level in facilities related to a nuclear reactor. However, due to the reason that the chemical property is almost not different from that of hydrogen, it is discharged under management to atmosphere or sea.
The highest value measured in Japan is 1,100 Bq/L which has been measured on Jun. 21, 2013 at the port of the first nuclear power plant of Fukushima where the nuclear disaster had happened. Since it is difficult for tritium to be separated chemically from hydrogen, a method for physical separation has been tried. However, it is still at a test level and practical success is yet to be made. Thus, radioactivity of the tritium discharged into the environment due to nuclear power plant disaster or the like cannot be removed with a current technology. The contaminated water containing tritium produced from the first nuclear power plant of Fukushima may reach 800,000 m3 or so in the future, and it is desired to have a method for effective treatment therefor as soon as possible.
Meanwhile, as the tritium concentration is at extremely low level, it is general to have electrolysis concentration for improving the measurement precision at the time of measuring the concentration. Herein, a method of preparing a sample solution containing dissolved electrolyte and performing electrolysis across a plate-like panel is known as the electrolysis concentration of heavy water of a related art. There is HDO or HTO as water included in an electrolyte solution in addition to H2O. They are decomposed into hydrogen and oxygen according to water electrolysis in general. However, due to the isotope effect, decomposition of H2O occurs prior to decomposition of HDO or HTO, and therefore as the concentration of deuterium or tritium increases in the electrolyte solution, concentration occurs. Nickel is used as an anode used for the electrolysis concentration. Steel, iron, nickel and the like are used as a cathode. Those electrodes are cleaned and sample water which is prepared by adding dilute sodium hydroxide as a support salt to water solution containing heavy water is added to a glass container. Then electrolysis is performed by applying electric current. At that time, while the current density is set at 1 to 10 A/dm2 or so and the solution temperature is kept at 5° C. or lower to prevent evaporation of water caused by heating, the electrolysis is generally continued until the liquid amount is reduced to 1/10 or so to have the concentration of deuterium.
Namely, the electrolysis concentration of tritium is based on the property that, like the case of deuterium, electrolysis of tritium water is more difficult than water with light hydrogen. Regarding the electrolysis method including insertion of a metal electrode into an aqueous alkali solution, various studies have been already made so that a standard method is present as an official manual. According to this method, tritium concentration is concentrated with 1 stage. However, in terms of an actual case, there are several problems in the electrolysis concentration of a related art, i.e., the operational works are complicated, tritium concentration rate is limited by the upper limit of electrolyte concentration, mixture gas of hydrogen and oxygen is produced to yield a risk of explosion, it takes long time for the electrolysis, and the method is not suitable for a large-scale treatment.
As the technology is determined from the viewpoint of separating and capturing a barely-contained material with 1 stage, the above problems are mainly caused by using an aqueous alkali solution electrolysis of a related art in which handling an alkali water electrolytic water solution is difficult, separating the gas generated from an anode is difficult, increasing electrolytic current is difficult due to forming of air bubbles on a metal surface, or the like.
In this regard, as an electrolysis method for water which receives attention in recent years, a water electrolysis using a solid polymer electrolyte (hereinbelow, referred to as “SPE”) can be mentioned (hereinbelow, referred to as “SPE water electrolysis”). The first SPE water electrolysis is made by General Electric Company of USA, by applying the technology of fuel cell in early 1970s. With regard to the structure of electrolysis part, both surfaces of a SPE membrane are sandwiched between porous metal electrodes, and by immersing them in pure water and just applying electric current, electrolysis is caused to release decomposed gas from the porous electrodes. SPE is a kind of a cation exchange resin, and it has a structure in which a sulfonic acid group or the like for having ion transport is chemically bound to a polymer chain. When electric current is applied between two electrodes, water is decomposed and oxygen gas is produced at the anode and hydrogen ions are produced. Those hydrogen ions are transported to the cathode after moving through the sulfonic acid groups of SPE, and after taking electrons, hydrogen gas is generated. Apparently, SPE itself does not undergo any change and is maintained in a solid phase.
In a case of using the SPE for electrolysis concentration of tritium, it is expected to have the following advantages compared to a method of a related art.
1) Distilled water can be directly decomposed. Namely, dissolution and neutralization of an electrolyte and removal of an electrolyte, which are essential in aqueous alkali solution electrolysis, are not necessary and the rate of volume decrease of sample water is limitless, in principle.
2) As the electrode surface is not covered by air bubbles, electrolysis can be carried out with high electric current and thus the time for electrolysis can be shortened.
3) As the hydrogen gas and oxygen gas are separately produced at different sides of a SPE membrane, gas treatment is easy, and it is much safer than a method of a related art in which explosive mixture gas is handled.
Furthermore, regarding the electrolysis concentration method of heavy water based on SPE water electrolysis, there are Patent Literatures 1 and 2 suggested by the applicant company and Non Patent Literature 1.
However, in a case of using Patent Literatures 1 and 2 and Non Patent Literature 1, an application can be made for a device for analysis or concentration of small scale, but they are not suitable for a treatment of large scale based on the following reasons. Since an electrolyte solution to be used is pure water, to have no flow of electric current in the electrolyte solution, the solid polymer membrane as a constitutional element needs to be strongly clamped at an anode and a cathode with surface pressure of 20-30 Kg/cm2 or so. As such, it is required for each member of an electrolysis bath to have high strength. However, having a large reaction area like 1 m2 or more is not practical when economic efficiency or operational property is considered. Also, they are not suitable for electrolysis concentration or fractionation of raw water containing large volume of heavy water due to high cost involved with facilities or the like.